On vdsl connection does DLM increase Latency?
It can, by turning on interleaving.
They increased latency by a minimum 8ms (low interleaving).
It's exactly 8ms that's added in that case. It's never variable.
If the line is really bad and has g.inp (retx) enabled them some retransmitted packets can cause very small delays randomly but this would generally be unnoticeable.
Can you describe your symptoms and it can be advised if it's likely DLM or not. Any line starts available to you would also help.